selftests: gpio-mockup-chardev: Check asprintf() for error

With gcc 7.3.0:

    gpio-mockup-chardev.c: In function ‘get_debugfs’:
    gpio-mockup-chardev.c:62:3: warning: ignoring return value of ‘asprintf’, declared with attribute warn_unused_result [-Wunused-result]
       asprintf(path, "%s/gpio", mnt_fs_get_target(fs));
       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Handle asprintf() failures to fix this.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Shuah Khan <shuah@kernel.org>

tools/testing/selftests/gpio/gpio-mockup-chardev.c

@@ -37,7 +37,7 @@ static int get_debugfs(char **path)
 	struct libmnt_table *tb;
 	struct libmnt_iter *itr = NULL;
 	struct libmnt_fs *fs;
-	int found = 0;
+	int found = 0, ret;
 
 	cxt = mnt_new_context();
 	if (!cxt)
@@ -58,8 +58,11 @@ static int get_debugfs(char **path)
 			break;
 		}
 	}
-	if (found)
-		asprintf(path, "%s/gpio", mnt_fs_get_target(fs));
+	if (found) {
+		ret = asprintf(path, "%s/gpio", mnt_fs_get_target(fs));
+		if (ret < 0)
+			err(EXIT_FAILURE, "failed to format string");
+	}
 
 	mnt_free_iter(itr);
 	mnt_free_context(cxt);